Exchange 5.5 Cant send mail to single domain

I ran into a problem I have a client that can not send to a single domain. I telneted to the recipients mail server from within my clients network and could relay an e-mail to the intended recipient in the other domain ( no this is not an open relay e-mail servers will relay to internal peeps from telnet). Neither of the mail servers are on blacklists. If it was a RDNS issue I would have gotten a lookup error when I telneted in and sent the manual e-mail with the...
(on the recipients server)
rcpt to:xxx@recipients.dom
mail from:xxx@client.dom <- would have had a host not found after this if RDNS.

When sending through exchange I imediatelty get this RDN back (not real rcpt or sender data)

Your message did not reach some or all of the intended recipients.
Subject: test
Sent: 2/13/2001 11:29 AM

The following recipient(s) could not be reached:
c=US;a= ;p=org;o=site;dda:SMTP=user@microsoft.com; on 2/13/2001 11:33 AM The e-mail system was unable to deliver the message, but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator.The MTS-ID of the original message is:c=US;a= ;p=org;l=MAILSERVER-010213172832Z-2385 MSEXCH:IMSrg:site:MAILSERVER 3902 (000B099C) Host Unknown

I can telnet to the recipients mail server using their domain name from MX record from the senders exchange server so I know its DNS is working correctly also.

Any help would be greatly appreciated.

Nevermind I found the answer to this. seems exchange sp4 enables rdns by default and the person they where trying to send to didnt have a PTR.